Method and apparatus for phase-controlling a load

ABSTRACT

A load control device may control the amount of power provided to an electrical load utilizing a phase control signal that operates in a reverse phase control mode, a center phase control mode, and a forward phase control mode. A load control device may be configured to determine that the electrical load should be operated via a phase control signal operating in a forward phase-control mode. After determining to operate the electrical load via the phase control signal in the forward phase-control mode, the load control device may provide the phase control signal in a reverse phase-control mode for a predetermined period of time to the electrical load, for example, to charge a bus capacitor of the electrical load. Subsequently, the load control device may be configured to switch the phase control signal to the forward phase-control mode and provide the phase control signal in the forward phase-control mode to the electrical load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/801,522, filed Mar. 13, 2013, which claims the benefit of ProvisionalU.S. Patent Application No. 61/616,460, filed Mar. 28, 2012, thedisclosure of which is hereby incorporated herein by reference.

BACKGROUND

Load control devices, such as dimmer switches and dimming modules, forexample, may be configured to control an amount of power provided froman alternating current (AC) power source to a load, such as a lightingload, for example. Such load control devices may employ a bidirectionalsemiconductor switch that is coupled in series electrical connectionbetween the AC power source and the load. The bidirectionalsemiconductor switch may be controlled to be conductive andnon-conductive for portions of a half-cycle of the AC power source, forexample, to control the amount of power delivered to the load (e.g.,using a phase-control dimming technique). For example, the bidirectionalsemiconductor switch may comprise one semiconductor switch, such as, butnot limited to a triac or a field effect transistor (FET) within afull-wave rectifying bridge; two semiconductor switches, such as, butnot limited to two FETs or two insulated gate bipolar transistors(IGBTs), coupled in anti-series electrical connection, or twosilicon-controlled rectifiers (SCRs) coupled in anti-parallel electricalconnection.

Load control devices may use a forward phase-control dimming techniqueor a reverse phase-control dimming technique, for example, to controlwhen the bidirectional semiconductor switch is rendered conductive andnon-conductive to control the power delivered to the load. Duringforward phase-control dimming, the bidirectional semiconductor switchmay be turned on at some point within each AC line voltage half-cycleand remains on until the next voltage zero crossing. Forward phasecontrol dimming may be used to control the power delivered to aresistive or inductive load, which may include, for example, anincandescent lamp or a magnetic low-voltage transformer, respectively.During reverse phase-control dimming, the bidirectional semiconductorswitch may be turned on at the zero crossing of the AC line voltage andturned off at some point within each half-cycle of the AC line voltage.Reverse phase-control dimming may be used to control the power deliveredto a capacitive load, which may include, for example, an electronic lowvoltage transformer. Given that the bidirectional semiconductor switchmay be rendered conductive at the beginning of the half-cycle and may beable to be turned off within the half-cycle, reverse phase controldimming may be utilized by a dimmer switch that includes two FETs inanti-serial connection, or the like.

Load control devices may be programmed by a user during installation touse the reverse phase-control dimming technique or the forwardphase-control dimming technique during operation. Alternatively, loadcontrol devices may employ a load detection process wherein the loadcontrol device may determine the type of load that it is controlling anduse the phase-control dimming technique that is best suited for thatload type. For example, a load control device may detect that the loadis inductive, and may determine to use the forward phase-control dimmingtechnique. For example, upon initial power up, such a load controldevice may begin using a reverse phase-control dimming technique (e.g.,operating in reverse phase-control mode) and may monitor the voltageacross the load during the load detection process. In the event that theload control device detects an overvoltage condition (e.g., a voltagespike), the load control device may then determine that the load hasinductive characteristics, and may accordingly begin using a forwardphase-control dimming technique (e.g., operating in forwardphase-control mode). After a load control device determines to employ aphase-control technique (e.g., forward phase-control dimming technique),for example, in response to a load detection process, user programming,or as a result of pre-configuration at the factory), the load controldevice may continue to use the determined phase-control technique duringoperation and may not deviate from using the determined phase-controltechnique.

However, a load control device may achieve improved performance as aresult of deviating from a phase-control technique (e.g., a forwardphase-control dimming technique) to employ another phase-controltechnique (e.g., a reverse phase-control dimming technique) duringcertain conditions. Therefore, there is a need for an improved loadcontrol device that is operable to employ one phase-control technique(e.g., a forward phase-control dimming technique) during operation andemploy another phase-control technique (e.g., a reverse phase-controldimming technique) during certain conditions.

Further, some electrical loads, such as a compact fluorescent lamp (CFL)or a light emitting diode (LED) lamp, for example, may comprise acapacitor (e.g., a bus capacitor). If the capacitor is not charged, thenthe use of a forward phase-control diming technique may cause a currentspike, which, for example, may occur at start-up when the capacitor isfully dissipated. This current spike may be due to the relatively largechange in voltage across the capacitor at a given time (e.g., aninstantaneous voltage across the capacitor). However, the load coupledto the load control device may be best suited for a forwardphase-control dimming technique. Therefore, there is a need for animproved load control device that may be operable to employ a reversephase-control dimming technique and/or a center phase control techniqueto charge the capacitor, and to employ a forward phase-control dimmingtechnique to operate the load.

Additionally, an electrical load, such as a light emitting diode (LED)lamp, for example, may require a certain minimum voltage across it inorder to turn on (e.g., emit light). However, once turned on, the loadmay be able to operate with an even lower voltage (and thus provide alower light intensity) than is required to turn the lamp on. Therefore,a load control device may be forced to select between operating a loadwith the lowest possible light output and guaranteeing that the loadwill turn on when the certain minimum voltage is being applied. Thus,there is a need for an improved load control device that is operable toemploy a reverse phase-control technique, a center phase-controltechnique, and a forward phase-control dimming technique in a singleoperation, for example, in order to charge a capacitor, providesufficient voltage across a load to turn it on, and operate the loadwhile allowing it to reach its lowest possible light output (low-end).

SUMMARY

A load control device may be configured to control an amount of powerdelivered from an alternating current (AC) power source to an electricalload. The load control device may include a bidirectional semiconductorswitch and a controller. The bidirectional semiconductor switch may beconfigured to be coupled between the AC power source and the electricalload. The controller may be operatively coupled to the bidirectionalsemiconductor switch. The controller may be configured to provide afirst line cycle of the phase control signal (e.g., to the electricalload), where the first line cycle is characterized by a first turn-onevent and a first conduction time. The controller may be configured toprovide a second line cycle of the phase control signal, where thesecond line cycle characterized by a second turn-on event and a secondconduction time. The second turn-on event may be greater than the firstturn-on event. The controller may be configured to provide a third linecycle of the phase control signal the third line cycle characterized bya third turn-on event and a third conduction time. The third turn-onevent may be greater than the second turn-on event, and the firstconduction time, the second conduction time, and the third conductiontime may be substantially the same. For example, the first phase controlsignal may be a reverse phase control signal, the second phase controlsignal may be a center phase control signal, and the third phase controlsignal may be a forward phase control signal.

A load control device may be configured to control an amount of powerdelivered from an AC power source to an electrical load. The loadcontrol device may include a bidirectional semiconductor switch and acontroller. The controller may be configured to provide a first linecycle of the phase control signal (e.g., to the electrical load), wherethe first line cycle is characterized by a first turn-on event and afirst Root Mean Squared (RMS) voltage value. The controller may beconfigured to provide a second line cycle of the phase control signal,where the second line cycle characterized by a second turn-on event anda second RMS voltage value. The second turn-on event may be greater thanthe first turn-on event. The controller may be configured to provide athird line cycle of the phase control signal the third line cyclecharacterized by a third turn-on event and a third RMS voltage value.The third turn-on event may be greater than the second turn-on event,and the first RMS voltage value, the second RMS voltage value, and thethird RMS voltage value may be substantially the same. For example, thefirst phase control signal may be a reverse phase control signal, thesecond phase control signal may be a center phase control signal, andthe third phase control signal may be a forward phase control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example load control device.

FIG. 2A is a diagram illustrating an example voltage waveform of aforward phase-control dimming signal.

FIG. 2B is a diagram illustrating an example voltage waveform of areverse phase-control dimming signal.

FIG. 2C is a diagram illustrating an example voltage waveform of areverse phase-control dimming signal with a minimum conduction time.

FIG. 3 is a simplified flowchart of an example start-up procedure.

FIG. 4 is a simplified flowchart of an example phase control modedetermination procedure.

FIG. 5 is a simplified flowchart of an example turn-on procedure.

FIG. 6 is a simplified block diagram of an example load control device.

FIG. 7 is a simplified block diagram illustrating an example of a dimmerswitch coupled to an LED lamp.

FIG. 8 is a simplified block diagram of an example load control device.

FIG. 9 is a diagram illustrating an example voltage waveform of adimming procedure comprising a phase control signal that operates inreverse phase control mode and forward phase control mode.

FIGS. 10A-10E are diagrams illustrating waveforms of an example dimmingprocedure.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of an example load control device(e.g., load control device 100). The load control device 100 may becoupled between a hot reference of an alternating current (AC) powersource 102 (e.g., 120 V, 60 Hz) and a load 104 via a hot terminal 106and a dimmed hot terminal 108, respectively. The load 104 may be coupledbetween the 108 terminal and a neutral reference of the AC power source102. The load 104 may be a lighting load, for example, an incandescentlighting load, a low voltage lighting load including a magnetic lowvoltage transformer, an electronic low voltage transformer, afluorescent light source, an LED light source, or any other suitabletype of lighting load. The load 104 may comprise a motor load, such as afan or a motorized window treatment. The load control device 100 maycomprise a neutral terminal 110, which may be coupled to the neutralreference of the AC power source 102.

The load control device 100 may comprise an airgap switch (e.g., a relay112), which may be coupled to the hot terminal 106 and may provide aswitched hot voltage 109 to a load control circuit 124. The load controlcircuit 124 may be coupled to the dimmed hot terminal and may beoperable to control the amount of power provided to and the intensity ofthe lighting load 104. The relay 112 and/or the load control circuit 124may be controlled by a microprocessor 114. The microprocessor 114 may beany suitable controller, for example, such as but not limited to aprogrammable logic device (PLD), a microcontroller, or an applicationspecific integrated circuit (ASIC). The microprocessor 114 may becoupled to a memory 115 for storage of data regarding the operation ofthe load control device 100. The memory 115 may be integral to themicroprocessor 114.

The load control circuit 124 may comprise a bidirectional semiconductorswitch, for example, two field effect-transistors (FETs) 126A, 126Bcoupled in anti-series electrical connection. The bidirectionalsemiconductor switch may control the amount of power delivered to theload 104. Each FET 126A, 126B may be coupled to a respective drivecircuit 128A, 128B, which may provide a gate voltage to each FET inorder to render each FET conductive. Each FET 126A, 126B may becontrolled individually during each half cycle of the AC power source102. Using a phase-control dimming technique, the microprocessor 114 maycontrol the drive circuits 128A, 128B to render the FETs 126A, 126Bconductive for a portion of each half cycle to provide power to the load104 and non-conductive for the other portion of the half cycle.

For example, during the positive half cycle, the drive circuit 128A mayprovide an active (e.g., high) gate voltage to FET 126A in order torender the FET conductive during a portion of the positive half cycle,and may remove the active gate voltage to FET 126A in order to renderthe FET non-conductive during the remaining portion of the positive thehalf cycle. During the negative half cycle, the drive circuit 128B mayprovide the active (e.g., high) gate voltage to FET 126B in order torender the FET conductive during a portion of the negative half cycle,and may remove the active gate voltage to FET 126B in order to renderthe FET non-conductive during the remaining portion of the negative thehalf cycle. Each FET 126A, 126B may be conductive for the same amount oftime within a full line cycle (e.g., the conduction time of FET 126A isequal to the conduction time of FET 126B for a given line cycle) inorder to provide a symmetric voltage waveform to the load 104.

The load control device 100 may comprise a zero-crossing detector 120that may determine the zero-crossings of the line voltage of the ACpower source 102. A zero-crossing may include the time at which the linevoltage transitions from a positive polarity to a negative polarity,and/or from a negative polarity to a positive polarity, for example, atthe beginning of each half-cycle. The zero-crossing information may beprovided as an input to the microprocessor 114. The microprocessor 114may control the FETs 126A, 126B to be conductive and non-conductive atpredetermined times relative to the zero-crossing points of the ACwaveform using a phase-control dimming technique.

The zero-crossing detector 120 may comprise an active filter forreceiving the line voltage from the AC power source 102, for example, tofilter any noise produced by other electrical devices and for recoveringthe AC fundamental waveform. The recovered AC fundamental may besubstantially free of noise or distortion, and of frequency componentsgreater than at least second order harmonics that may be present on theline voltage of the AC power source 102, which might otherwise result infaulty or incorrect zero crossing detection. The filter may take ananalog or digital (e.g., software) form and may be described in greaterdetail in commonly-assigned U.S. Pat. No. 6,091,205, issued Jul. 18,2000, and commonly-assigned U.S. Pat. No. 6,380,692, issued Apr. 30,2002, both entitled PHASE CONTROLLED DIMMING SYSTEM WITH ACTIVE FILTERFOR PREVENTING FLICKERING AND UNDESIRED INTENSITY CHANGES. The entiredisclosures of both patents are hereby incorporated by reference.

The load control device 100 may comprise a voltage compensation circuit122. The voltage compensation circuit 122 may integrate a signalrepresentative of a square of an amplitude of the electrical AC waveformto produce a signal representative of the energy delivered to the load104 in a given half-cycle. The voltage compensation circuit 122 mayprovide that signal to the microprocessor 114. For example, if areverse-phase control dimming technique is being used, themicroprocessor 114 may use the signal generated by the voltagecompensation circuit 122 to control the load control circuit 124 inresponse to the energy delivered to the load 104. The voltagecompensation circuit 122 may be described in greater detail incommonly-assigned U.S. Pat. No. 7,259,524, issued Aug. 21, 2007,entitled APPARATUS AND METHODS FOR REGULATING DELIVERY OF ELECTRICALENERGY, the entire disclosure of which is hereby incorporated byreference.

The load control device 100 may include a communication circuit 116 thatmay be coupled to the microprocessor 114. The microprocessor 114 may beoperable to send and/or receive digital control signals via thecommunication circuit 116, which may be coupled to a communication link.The communication link may comprise a low-voltage wired link, or awireless link, for example, such as but not limited to a radio frequency(RF) or an infrared (IR) communication link. For example, a plurality ofremote control devices (not shown) may be coupled to the communicationlink. A remote control device (e.g., each remote control device) may beoperable to send a digital control signal to the load control device 100to provide for control of the load 104. A power supply 118 may becoupled between the hot terminal 106 and the neutral terminal 110. Thepower supply 118 may generate a direct-current (DC) voltage VCC (notshown) for powering the microprocessor 114, the communication circuit116, and/or other low voltage circuitry of the load control device 100.For example, the load control device 100 may not require a connection tothe neutral side of the AC power source 102 and may not include aneutral terminal N. The power supply 118, the voltage compensationcircuit 122, and the zero-crossing detector 120 may be referenced to thedimmed hot terminal 108 instead of the neutral terminal. For example,the load control device 100 may include a user interface (not shown)coupled to the microprocessor 114, for example, such that the loadcontrol device may be readily controlled and monitored by a user.

The load control device 100 may comprise a voltage monitor circuit 130and/or a current monitor circuit 132 which may form part of the loadcontrol circuit 124. The voltage monitor circuit 130 may be coupledacross the FETs 126A, 126B, and may comprise a full-wave op-amprectifying circuit (not shown). The voltage monitor circuit 130 maydetect the voltage across the FETs 126A, 126B and provide an outputsignal representative of this detected voltage to the microprocessor114. The microprocessor 114 may be operable to use this output signalfrom the voltage monitor circuit 130 to determine whether the load 104is inductive, for example, as described herein. The microprocessor 114may use the output signal from the voltage monitor circuit 130 to verifythat the FETs 126A, 126B are operating properly (e.g., not shorted).

The current monitor circuit 132 may be coupled in series electricalconnection with the FETs 126A, 126B. The current monitor circuit 132 maydetermine the magnitude of the current flowing through the FETs andprovide an output signal representative of this current to themicroprocessor 114. The microprocessor 114 may be operable to use thisoutput signal from the current monitor circuit 132 to detect anover-current condition. For example, such an over-current condition maybe caused by improper wiring of the load control device 100 (e.g., amis-wiring of the 108 terminal to the neutral reference of the AC powersource 102) or by the load 104 failing in a shorted state. Themicroprocessor 114 may be operable to turn off the FETs in response todetecting an over-current condition, for example, to protect the FETs126A, 126B from potential damage that may result from such over-currentconditions (e.g., so the FETs are not exposed to excess current).

FIG. 2A is a diagram illustrating an example voltage waveform of aforward phase-control dimming signal. FIG. 2B is a diagram illustratingan example voltage waveform of a reverse phase-control dimming signal. Aload control device (e.g., load control device 100) may generate aphase-control voltage V_(PC) (e.g., a dimmed-hot voltage) at the dimmedhot terminal, for example, to control the intensity of a load (e.g.,load 104). The phase-control voltage V_(PC) may comprise a forwardphase-control waveform, for example, a leading edge phase controlvoltage (e.g., as shown in FIG. 2A) when the load control device isusing the forward phase-control dimming technique. The phase-controlvoltage V_(PC) may comprise a reverse phase-control waveform, forexample, a trailing edge phase-control voltage (e.g., as shown in FIG.2B) when the load control is using the reverse phase-control voltage.

For example, if the load control device is using the forwardphase-control dimming technique, a microprocessor of the load controldevice (e.g., microprocessor 114) may be operable to control abidirectional semiconductor switch of the load control device (e.g.,FETs 126A, 126B) to be non-conductive at the beginning of each halfcycle for an off period T_(OFF) and to control the bidirectionalsemiconductor switch to be conductive for a conduction period T_(CON),which, for example, may last the remainder of each half cycle. If theload control device is using the reverse phase-control dimmingtechnique, the microprocessor may be operable to control thebidirectional semiconductor switch to be conductive at the beginning ofeach half cycle for the conduction period T_(CON) and to control theFETs to be non-conductive for the off period T_(OFF). The load controldevice may adjust the conduction period T_(CON), for example, when usinga phase-control dimming technique. For example, if the load controldevice increases the conduction period T_(CON), then the amount of powerprovided from an AC power source (e.g., AC power source 102) to the loadmay increase, which may increase the lighting intensity of the load.Similarly, if the load control device decreases the conduction periodT_(CON), then the amount of power provided from an AC power source(e.g., AC power source 102) to the load may decrease, which may decreasethe lighting intensity of the load.

FIG. 2C is a diagram illustrating an example voltage waveform of areverse phase-control dimming signal with a minimum conduction time. Amicroprocessor (e.g., microprocessor 114) of a load control device(e.g., load control device 100) may be operable to monitor the currentthrough a bidirectional semiconductor switch (e.g., FETs 126A, 126B),for example, via a current monitor circuit (e.g., current monitorcircuit 132), and detect an over-current condition. Although themicroprocessor may be operable to detect such an over-current condition,the current monitor circuit may require that the bidirectionalsemiconductor switch be conductive in order to monitor the currentflowing through it. If there is an over-current condition, any exposureto excessive currents may damage the bidirectional semiconductor switch,and thus, such exposure should be limited. The bidirectionalsemiconductor switch may be initially rendered conductive for a minimumconduction period T_(CON-MIN) (e.g., approximately 1 ms) when firstturning the load (e.g., load 104) on.

By using the minimum conduction period T_(CON-MIN), the magnitude of anypossible excess current may be limited along with the duration of anypossible over-current exposure. Using the minimum conduction periodT_(CON-MIN) in conjunction with a forward phase-control dimmingtechnique may be insufficient to fully protect the bidirectionalsemiconductor switch from permanent damage in the event of anover-current condition. For example, the microprocessor and the currentmonitor circuit may not be able to respond quickly enough to thedetected over-current condition to sufficiently protect thebidirectional semiconductor switch (e.g., by turning off thebidirectional semiconductor switch).

The minimum conduction period T_(CON-MIN) may be used in conjunctionwith the reverse phase-control dimming technique, for example, as shownin FIG. 2C, such that the bidirectional semiconductor switch may berendered conductive at the very beginning of each half cycle, forexample, when the magnitude of voltage of an AC power source (e.g., ACpower source 102) is zero volts or nearly zero volts. Thus, themagnitude of any excess current may be at its lowest possible magnitude,and the current monitor circuit may detect the indication of anover-current condition to limit the overall exposure of thebidirectional semiconductor switch to such excessive currents.

The inrush current of a load (e.g., an incandescent load having a coldtungsten filament) may be detected by the current monitor circuit as anover-current condition. An over-current condition caused by an inrushcurrent may not be considered to be a fault condition. In the event ofnormal inrush current, the bidirectional semiconductor switch of theload control device may not need the protection that is required in theevent of excessive current caused from a fault condition. Providing suchprotection of the bidirectional semiconductor switch in response to aninrush current may be undesirable, for example, as it may impede theoperation of the load control device. The load control device maydistinguish between an over-current condition caused by a faultcondition and an over-current condition caused by inrush current. Forexample, the microprocessor of the load control device may detect thisdistinction, for example, by monitoring the rate of change as well asthe total magnitude of the current flowing through the bidirectionalsemiconductor switch. When using the forward phase-control dimmingtechnique, the inductance of the power wiring (e.g., line inductance)may act to limit the rate of change of current flowing through thebidirectional semiconductor switch, which, for example, may make itdifficult to distinguish between an over-current condition caused by thefault condition and an over-current condition caused by an inrushcurrent. When using the reverse phase-control dimming technique, theeffect of the line inductance may be minimized. The microprocessor maymake this distinction more reliably when the bidirectional semiconductorswitch is initially rendered conductive for a minimum conduction periodT_(CON-MIN) using the reverse phase-control dimming technique instead ofthe forward phase-control technique.

FIG. 3 is a simplified flowchart of an example procedure (e.g., start-upprocedure) executed by a microprocessor. The start-up procedure 200 maybe executed by a microprocessor (e.g., microprocessor 114) when power isfirst applied to a load control device (e.g., load control device 100).For example, the start-up procedure 200 may be executed by themicroprocessor following a reset event. The start-up procedure 200 mayprovide for the load control device to use a reverse phase-control (RPC)dimming technique (e.g., to operate in a reverse phase-control mode) fora predetermined period of time T_(RPC). For example, the predeterminedperiod of time may be two lines cycle periods of an AC power source(e.g., AC power source 102). After using a reverse phase-control dimmingtechnique for the predetermined period of time T_(RPC), the start-upprocedure 200 may provide for the load control device to use a forwardphase-control (FPC) dimming technique (e.g., operating in a forwardphase-control mode).

Referring to FIG. 3, at 202 the microprocessor may determine whether theload control device should stay in the on state, for example, bychecking the contents of a memory (e.g., the memory 115). If it isdetermined that the load control device should be on, then themicroprocessor may initialize a timer to the predetermined period oftime T_(RPC) at 206. If it is determined that the load control deviceshould not be in the on state, then the microprocessor may wait for acommand to turn on at 204 until proceeding to 206.

The microprocessor may render a bidirectional semiconductor switch(e.g., the FETs 126A, 126B) conductive, for example, using the minimumconduction period T_(CON-MIN) and the reverse phase-control dimmingtechnique at 208. The microprocessor may sample the output signalI_(SMP) from a current monitor circuit (e.g., current monitor circuit132), for example, which may be representative of the magnitude of thecurrent flowing through the bidirectional semiconductor switch. Forexample, the microprocessor may look for evidence of an over-currentcondition. The microprocessor may continue to sample the output signalI_(SMP) while driving the bidirectional semiconductor switch conductivefor the minimum conduction period T_(CON-MIN), for example, until thepredetermined period of time T_(RPC) expires at 214 or the sampledoutput signal I_(SMP) reaches or exceeds a maximum current thresholdI_(MAX) at 210. If the microprocessor determines that the sampled outputsignal I_(SMP) exceeds the maximum current threshold I_(MAX) (e.g., 56A), then the microprocessor may enter an over-current protection mode at212 before the start-up procedure 200 exits.

During the over-current protection mode 212, the microprocessor mayturn-off the bidirectional semiconductor switch (e.g., initially), andmay analyze the rate of change of the sampled output signal I_(SMP) todetermine whether the over-current condition may have been caused bynormal inrush current or a true fault condition. If the microprocessordetermines that the rate of change of the sampled output signal I_(SMP)is indicative of normal inrush current during the over-currentprotection mode 212, then the microprocessor may render thebidirectional semiconductor switch conductive in the following linecycle while continuing to monitor the current via the current monitoringcircuit.

If the microprocessor does not detect an over-current condition and thepredetermined period of time T_(RPC) expires at 214, then themicroprocessor may perform a phase control mode determination process(e.g., a phase control mode determination process 300 as shown in FIG.4), for example, to determine whether the load control device should usethe reverse phase-control dimming technique or the forward phase-controldimming technique. At 216, the microprocessor may check whether the loadcontrol device should operate in the forward phase-control (FPC) modeand if so, the microprocessor may control the bidirectionalsemiconductor switch using the forward phase-control dimming techniqueat 218. For example, the microprocessor may adjust the conduction timeT_(CON) of the bidirectional semiconductor switch to achieve a desiredpreset lighting intensity L_(PRE) of a lighting load (e.g., lightingload 104). The microprocessor may employ a fading technique to graduallyincrease the lighting intensity of the lighting load to the presetintensity L_(PRE). If it is determined that the microprocessor shouldnot operate in the forward-phase control mode at 216, then themicroprocessor may continues to use the reverse phase-control (RPC)dimming technique, and may control the bidirectional semiconductorswitch to achieve the preset lighting intensity L_(PRE) of the lightingload at 220.

FIG. 4 is a simplified flowchart of a phase control mode determinationprocess executed by a microprocessor. A microprocessor (e.g.,microprocessor 114) may determine whether the operating mode (e.g.,reverse phase-control mode or forward phase-control mode) has beenprogrammed or selected by a user, for example, after installation of aload control device (e.g., load control device 100) at 302. If themicroprocessor determines that the operating mode has been selected bythe user, then the microprocessor may check whether the reversephase-control mode has been selected at 304. If the reversephase-control mode has been selected at 304, then the microprocessor maydetermine at 306 that the load control device should use the reversephase-control mode for its subsequent operation. The microprocessor maysave this mode information in memory, for example, in local memoryinternal to the microprocessor or in a memory (e.g., memory 115). If at304, the reverse phase-control mode has not been selected, then themicroprocessor may determine that the load control device should useforward phase-control mode at 308 for its subsequent operation.

If the microprocessor determines that the operating mode has not beenselected by the user at 302, then at 310 the microprocessor maydetermine whether it is capable of automatically detecting the type ofload (e.g., load 104) to which the load control device is coupled. Ifthe microprocessor is capable of detecting the load type, then themicroprocessor may determine whether the load has inductivecharacteristics at 312. If the microprocessor determines that the loadhas inductive characteristics, then the microprocessor may determinethat the load control device should use forward phase-control mode at308 for its subsequent operation. If the microprocessor determines thatthe load does not have inductive characteristics, then themicroprocessor may proceed to step 306 to determine that the loadcontrol device should use reverse phase-control mode for its subsequentoperation.

If the microprocessor is not capable of detecting the load type, thenthe microprocessor may determine whether the load control device hasbeen preconfigured (e.g., factory programmed) to operate in the reversephase-control mode at 314. If so, then the microprocessor may determinethat the load control device should use the reverse phase-control modefor its subsequent operation at 306. Otherwise, the microprocessor maydetermine at 308 that the load control device should use forwardphase-control mode for its subsequent operation.

FIG. 5 is a simplified flowchart of a turn-on procedure executed by amicroprocessor during normal operation. For example, the turn-onprocedure 400 may be executed by a microprocessor (e.g., microprocessor114) of a load control device (e.g., load control device 100) inresponse to receiving a command to transition from an electronic offstate (e.g., when the microprocessor is powered, but a load is off) toan electronic on state. The turn-on procedure 400 may comprise one ormore of decisions utilized by the start-up procedure 200 (e.g., as shownin FIG. 3). For example, the turn-on procedure 400 may not comprise 202(e.g., checking the on state) and 204 (e.g., waiting for an on command)of the start-up procedure 200, for example, because the turn-onprocedure 400 may be initiated in response to having received a commandto turn on the load (e.g., load 104). The turn-on procedure 400 may notcomprise the phase control mode determination procedure 300 that may beused during the start-up procedure 200. The microprocessor may notre-determine the phase control mode with every transition fromelectronic off to on. The microprocessor may rely upon the mode that waspreviously determined during the start-up procedure 200 at initialpower-up. The turn-on procedure 400 may comprise the phase control modedetermination procedure 300. 406-420 of the turn-on procedure 400 mayprovide for the load control device 100 to use a reverse phase-controldimming technique (e.g., to operate in a reverse phase-control mode) fora predetermined period of time T_(RPC) before using a forwardphase-control dimming technique (e.g., operating in a forwardphase-control mode) during every transition from the electronic offstate to the on state during operation. For example, the predeterminedperiod of time T_(RPC) may be two lines cycle periods of an AC powersource (e.g., AC power source 102).

FIG. 6 is a simplified block diagram of a load control device. Loadcontrol device 500 may comprise one or more (e.g., four) load controlcircuits 524 to control (e.g., individually control) the one or more(e.g., four) loads 504, for example, via one or more (e.g., four) dimmedhot terminals DH1, DH2, DH3, DH4. The loads 504 may be lighting loads. Amicroprocessor 514 may be operable to control each load control circuit524 in the same manner as microprocessor 114 of load control device 100.One or more of the functional blocks of load control device 500 may besubstantially the same as the functional blocks of the load controldevice 100.

The dimming techniques described herein may be beneficial whenperforming an over-current detection (e.g., as described herein), butsuch a dimming technique may have additional applications. For example,a load control device (e.g., load control device 100 or load controldevice 500) may use the reverse phase-control dimming technique for apredetermined period of time before using the forward phase-controldimming technique during every transition from the electronic off stateto the on state to improve the dimming performance of certain lightingload types. For example, some lighting loads, such as but not limited tocompact fluorescent lamps and lighting emitting diodes (LEDs), may bedesigned by a lighting manufacturer to be dimmed with a forwardphase-control dimming technique. Accordingly, a load control device mayuse the forward phase-control dimming technique during operation whendimming such loads. However, such lighting loads may also have acapacitive front end, and thus, it may be advantageous to use thereverse phase-control dimming technique for a predetermined period oftime when the load is transitioning from off to on. Further, when usingthe reverse phase-control dimming on such loads, it may be advantageousto use a conduction time T_(CON) that is greater than the minimumconduction period T_(CON-MIN). For example, when the load istransitioning from off to on, it may be advantageous to use a peak ofline conduction time T_(CON-PK) (e.g., having approximately a 90 degreephase angle) such that the FETs are rendered non-conductive near thepeak of the AC line voltage when using the reverse phase-control dimmingtechnique for the predetermined period of time T_(RPC). Thereafter, theload control device may use the forward phase-control dimming technique,and may subsequently use the minimum conduction period T_(CON-MIN) andthen gradually increase the conduction time of the FETs over multipleline cycles to smoothly adjust the intensity of the lighting load to thedesired preset lighting intensity L_(PRE). This may allow the capacitivefront end of these certain load types to charge quickly (e.g., whenusing the reverse phase-control dimming technique) before the loadcontrol device begins to control the intensity of the lighting load(e.g., when using the forward phase control dimming technique).

FIG. 7 illustrates an example of a dimmer switch coupled to an LED lamp.A dimmer switch 710 may include a hot terminal 702 that is coupled to anAC power source 720, and a dimmed hot terminal 704 that is coupled to aload (e.g., an LED lamp 730). The LED lamp 730 may comprise an LEDdriver 735 and an LED light source 740. The LED lamp 730 may be coupledto the dimmed hot terminal 704 and the neutral connection of the ACpower source 720. The LED driver 735 of the LED lamp 730 may furthercomprise a bus capacitor 736 adapted to be coupled to the dimmed hotterminal 704 of the dimmer switch 710 and the neutral connection of theAC power source 720. The dimmer switch 710 may be operable to providethe dimmed hot voltage using different phase control types, for example,forward phase control and reverse phase control. In addition, the dimmerswitch 710 may be operable to provide a full conduction voltage to theLED lamp 730.

The dimmer switch 710 may include a tap switch 714 for turning the LEDlight source 740 on and off, and a dimming rocker 716 which may be usedto adjust the intensity of the LED light source 740 (e.g., increase anddecrease the intensity by tapping the upper 716A and lower 716B portionsof the dimming rocker 716, respectively). The dimmer switch 710 mayinclude a controllably conductive device (e.g., one or morebidirectional semiconductor switches) that are operable to control theamount of power provided to the LED lamp 730 from the AC power source720 via a phase control signal. For example, the bidirectionalsemiconductor switch of dimmer switch 710 may be implemented as twofield effect transistors (FETs) in anti-series connection or a singleFET inside a full-wave rectifying bridge.

FIG. 8 is a simplified block diagram of a load control device. Loadcontrol device 800 may comprise a power supple 802, a controller 804(e.g., a microprocessor), a zero-crossing detector 806, a voltagecompensation circuit 808, a control actuator 810, an intensityadjustment actuator 812, a memory 818, a bidirectional semiconductorswitch 824, a hot terminal 814, and a dimmed-hot terminal 816. The loadcontrol device 800 may be, for example, a two wire dimmer. The loadcontrol device 800 may be similar to the dimmer switch 710.

The load control device 800 may be utilized to control an amount ofpower delivered to a load. The load may be a lighting load, for example,an incandescent lighting load, a low voltage lighting load including amagnetic low voltage transformer, an electronic low voltage transformer,a fluorescent light source, a compact fluorescent lamp (CFL), an LEDlamp, or any other suitable type of lighting load or combinationthereof. The load 104 may comprise a motor load, such as a fan or amotorized window treatment.

The bidirectional semiconductor switch 824 may be operably coupled inseries electrical connection between the hot terminal 814 and the dimmedhot terminal 816, for example, to control the power delivered to theload. The bidirectional semiconductor switch 824 may comprise two fieldeffect transistors (FETs) 826A, 826B in anti-series connection and twodrive circuits 828A, 828B. The bidirectional semiconductor switch 824may be implemented as a single FET inside a rectifying bridge. Thebidirectional semiconductor switch 824 may be a set of anti-series IGBTswith corresponding body diodes. The bidirectional semiconductor switch824 may provide a reverse phase control signal, a center phase controlsignal, a forward phase control signal, or a full conduction signal to aload as needed.

The controller 804 may be operably coupled to the bidirectionalsemiconductor switch 824. The controller 804 may be coupled to thebidirectional semiconductor switch 824 for rendering the bidirectionalsemiconductor switch conductive and nonconductive. The controller 804may be configured to control the bidirectional semiconductor switch 824in response to a zero-crossing detector 806. The zero-crossing detector806 may be configured to determine the zero-crossings of an input ACwaveform from an AC power source via the hot terminal 814. Thezero-crossing detector 806 may be similar to the zero-crossing detector120 described herein. Similarly, the voltage compensation circuit 808may be similar to the voltage compensation circuit 122 described herein.

The controller 804 may receive an input from the control actuator 810and/or the intensity adjustment actuator 812. The control actuator 810may be operable to allow for turning on and off the load. The controlactuator 810 may be, for example, an airgap switch, a relay, etc. Theintensity adjustment actuator 812 may be operable to allow a user toadjust the amount of power being delivered to the load. For example, theintensity adjustment actuator 812 may be operable to allow for theadjustment of a lighting intensity of the load, for example, from alow-end intensity setting to a high-end intensity setting. The intensityadjustment actuator 812 may be a slide actuator, a rotary knob, etc.

The controller 804 may be any suitable controller or microprocessor, forexample, such as but not limited to a programmable logic device (PLD), amicrocontroller, or an application specific integrated circuit (ASIC).The controller 804 may be coupled to the memory 818 for storage of dataregarding the operation of the load control device 800. The memory 818may be integral to the controller 804.

FIG. 9 is a diagram illustrating an example voltage waveform of adimming procedure. The waveform 900 may comprise a phase control signalthat operates in reverse phase control mode and forward phase controlmode. The voltage waveform 900 may be a representative phase controlsignal utilized during the dimming procedure 200 of FIG. 3, for example,where the predetermined period of time T_(RPC) of the start-up procedure200 is defined as the first two half-line cycle periods of an AC powersource (e.g., when the predetermined period of time T_(RPC) is equal toapproximately 0.017 sec).

A load control device (e.g., load control device 800) may determine tooperate a load using a phase control signal in a forward phase controlmode. After determining to operate the load using a phase control signalin a forward phase control mode, the load control device may provide aphase control signal utilizing a reverse phase-control dimming techniqueto the load for a predetermined period of time, for example, in responseto receiving a command to turn the load on. For example, thepredetermined period of time may be the first two half-line cycles ofthe phase control signal (e.g., 0.017 sec). Thereafter, the load controldevice may provide the phase control signal utilizing a forwardphase-control dimming technique for the following half-line cycles. Thefirst two half-line cycles utilizing a reverse phase-control dimmingtechnique may be used to charge a capacitor (e.g., a bus capacitor) ofthe load (e.g., the LED driver 735 of the lamp 730). Thereafter, theload control device may utilize a forward phase-control dimmingtechnique, for example, to operate the load.

The conduction time T_(CON-R) of the first half-line cycle of reversephase-control may be equal to the conduction time T_(CON-F1) of thefirst half-line cycle of forward phase-control. However, the conductiontime T_(CON-R) of the first half-line cycle of reverse phase-control mayor may not be equal to the conduction time of subsequent half-linecycles of forward phase-control. For example, the conduction timeT_(CON-R) may be equal to the conduction time T_(CON-F2) of the secondhalf-line cycle of forward phase-control. However, the conduction timeT_(CON-R) may not be equal to the conduction time T_(CON-F2), forexample, in a ramp-up start procedure where T_(CON-F1) may be less thanT_(CON-F2).

Similarly, the Root Mean Squared (RMS) voltage V_(RMS-R) of the firsthalf-line cycle of reverse phase-control may be equal to the RMS voltageV_(RMS-F1) of the first half-line cycle of forward phase-control.However, the RMS voltage V_(RMS-R) of the first half-line cycle ofreverse phase-control may or may not be equal to the RMS voltage ofsubsequent half-line cycles of forward phase-control. For example, theRMS voltage V_(RMS-R) may be equal to the RMS voltage V_(RMS-F2) of thesecond positive half-line cycle of forward phase-control. However, theRMS voltage V_(RMS-R) may not be equal to the RMS voltage V_(RMS-F2),for example, in a ramp-up start procedure where the RMS voltageV_(RMS-F1) of the first half-line cycle of forward phase-control may beless than the RMS voltage V_(RMS-F2) of the second positive half-linecycle of forward phase-control.

FIGS. 10A-10E are diagrams illustrating waveforms of an example dimmingprocedure. FIG. 10A is a diagram illustrating a waveform of aphase-control voltage V_(PC-1) operating according to a reversephase-control mode. FIG. 10B is a diagram illustrating a waveform of aphase-control voltage V_(PC-2) operating according to an off-centerphase-control mode. FIG. 10C is a diagram illustrating a waveform of aphase-control voltage V_(PC-3) operating according to an on-centerphase-control mode. FIG. 10D is a diagram illustrating a waveform of aphase-control voltage V_(PC-4) operating according to an off-centerphase-control mode. FIG. 10E is a diagram illustrating a waveform of aphase-control voltage V_(PC-5) operating according to a forwardphase-control mode.

A phase-control signal according to the example dimming procedure maycomprise a first line cycle as represented by the phase-control voltageV_(PC-1) of FIG. 10A, a second line cycle as represented by thephase-control voltage V_(PC-2) of FIG. 10B, a third line cycle asrepresented by the phase-control voltage V_(PC-3) of FIG. 10C, a fourthline cycle as represented by the phase-control voltage V_(PC-4) of FIG.10D, and a fifth line cycle as represented by the phase-control voltageV_(PC-5) of FIG. 10E, for example, in the order as described. Theexample dimming procedure of FIGS. 10A-10E may be performed by a loadcontrol device (e.g., load control device 800), and may be performed inresponse to receiving a command to transition from an electronic offstate (e.g., when the load control device 800 is powered, but a load isoff) to an electronic on state. A phase-control signal may comprise noneor any number of the phase control voltages V_(PC-1)-V_(PC-5). A phasecontrol voltage may comprise additional phase control voltages. Forexample, a phase control voltage may comprise a plurality of AC linecycles operating in forward phase control mode after providingphase-control voltage V_(PC-5) to a load.

Referring to FIG. 10A, a phase control signal operating in reverse phasecontrol mode is illustrated. The phase control signal of FIG. 10A may becharacterized by the phase-control voltage V_(PC-1). The phase controlsignal of FIG. 10A may be referred herein as the phase-control voltageV_(PC-1). The phase-control voltage V_(PC-1) may be characterized by aturn-on time t_(on1) and a turn-off time t_(off1). The turn-on time(turn-on event) t_(on1) may occur at, or substantially at, a zerocrossing of a positive half-line cycle of the AC line voltage. Forexample, the turn-on time t_(on1) may be substantially at the zerocrossing of the positive half-line cycle of the AC line voltage whenlow-end (e.g., lowest 1-5%) and/or high-end portions (e.g., highest1-5%) of the half-cycle of the AC line voltage are not utilized, forexample, in some load control devices utilized for dimming lightingloads. The turn-off time torn may occur during the positive half-linecycle of the AC-line voltage and before the negative zero-crossing ofthe AC-line voltage. For example, the turn-off time torn of thephase-control voltage V_(PC-1) may be before (e.g., as shown in FIG.10A), at, or after a midpoint 1000 of the positive half-cycle of AC-linevoltage.

The phase-control voltage V_(PC-1) may be characterized by a conductiontime T_(CON1). The conduction time T_(CON1) may correspond to the timewithin the positive half-cycle of the AC-line voltage when the loadcontrol device is providing voltage to the load. For example, theconduction time T_(CON1) may correspond to the time when a bidirectionalsemiconductor switch of the load control device is rendered conductive.The phase-control voltage V_(PC-1) may be characterized by a RMS voltagevalue V_(RMS-1). The RMS voltage value V_(RMS-1) may correspond to themagnitude of RMS voltage provided by the positive half-cycle of theAC-line voltage during the conduction time T_(CON1).

Referring to FIG. 10B, a phase control signal operating in off-centerphase control mode is illustrated. The phase control signal of FIG. 10Bmay be characterized by the phase-control voltage V_(PC-2). The phasecontrol signal of FIG. 10B may be referred herein as the phase-controlvoltage V_(PC-2). The phase-control voltage V_(PC-2) may becharacterized by a turn-on time tong and a turn-off time t_(off2). Theturn-on time t_(on2) may occur after a zero crossing of a positivehalf-line cycle of the AC line voltage, and before (e.g., as shown inFIG. 10B) or after a midpoint 1001 of the positive half-line cycle ofthe AC line voltage. The turn-off time t_(off2) may occur during thepositive half-line cycle of the AC-line voltage and before the negativezero-crossing of the AC-line voltage. For example, the turn-off timet_(off2) of the phase-control voltage V_(PC-2) may occur before, at, orafter (e.g., as shown in FIG. 10B) the midpoint 1001 of the positivehalf-cycle of AC-line voltage.

With respect to a positive half-line cycle of AC-line voltage, theturn-on time t_(on2) of the phase-control voltage V_(PC-2) may occurafter the turn-on time t_(on1) of the phase-control voltage V_(PC-1) ofFIG. 10A, but may occur before the turn-off time t_(off1) of thephase-control voltage V_(PC-1) of FIG. 10A. Therefore, the phase-controlvoltage V_(PC-2) of FIG. 10B may be said to overlap the phase-controlvoltage V_(PC-1) of FIG. 10A even though they may occur at differenttimes and in different line-cycles of the AC-line voltage. However, thephase-control voltage V_(PC-2) of FIG. 10B may not overlap thephase-control voltage V_(PC-1) of FIG. 10A.

The phase-control voltage V_(PC-2) may be characterized by a conductiontime T_(CON2). The conduction time T_(CON2) may correspond to the timewithin the positive half-cycle of the AC-line voltage when the loadcontrol device is providing voltage to the load. For example, theconduction time T_(CON2) may correspond to the time when a bidirectionalsemiconductor switch of the load control device is rendered conductive.The phase-control voltage V_(PC-2) may be characterized by a RMS voltagevalue V_(RMS-2). The RMS voltage value V_(RMS-2) may correspond to themagnitude of RMS voltage provided by the positive half-cycle of theAC-line voltage during the conduction time T_(CON2).

The phase-control voltage V_(PC-2) may be characterized as a phasecontrol signal operating in an off-center phase control mode because thephase-control voltage V_(PC-2) may comprise (e.g., overlap) the midpoint1001 of the positive half-line cycle of AC-line voltage, but themidpoint 1001 of the positive half-line cycle AC-line voltage may notsubstantially correspond to the halfway point of the conduction timeT_(CON2) (e.g., such an instance may be considered an on-center phasecontrol signal, for example, as described with reference to FIG. 10C).However, a phase control signal operating in an off-center phase controlmode may not comprise the midpoint 1001 of the positive half-line cycleof the AC line voltage at all. For example, a phase control signaloperating in an off-center phase control mode may be characterized by aturn-on time that is after the zero-crossing of a positive half-linecycle of the AC line voltage as the AC line voltage goes from positiveto negative, and a turn-off time that is before the midpoint 1001. Aphase control signal operating in an off-center phase control mode maybe characterized by a turn-on time that is after the zero-crossing ofthe AC line voltage as the AC line voltage goes from positive tonegative (e.g., and after the turn-on time of reverse phase controlsignal), and a turn-off time that is before next zero-crossing of the ACline voltage as the AC line voltage goes from negative to positive. Aphase control signal operating in an off-center phase control mode isnot a forward phase control signal, a reverse phase control signal, or afull conduction control signal.

Referring to FIG. 10C, a phase control signal operating in an on-centerphase control mode is illustrated. The phase control signal of FIG. 10Cmay be characterized by the phase-control voltage V_(PC-3). The phasecontrol signal of FIG. 10C may be referred herein as the phase-controlvoltage V_(PC-3). The phase-control voltage V_(PC-3) may becharacterized by a turn-on time t_(on3) and a turn-off time t_(off3).The turn-on time t_(on3) may occur after a zero crossing of a positivehalf-line cycle of the AC line voltage but before a midpoint 1002 of thepositive half-line cycle of the AC line voltage. The turn-off timet_(off3) may occur after the midpoint and during the positive half-linecycle of the AC-line voltage, but before the negative zero-crossing ofthe AC-line voltage.

With respect to a positive half-line cycle of AC-line voltage, theturn-on time t_(on3) of the phase-control voltage V_(PC-3) may occurafter the turn-on time tong of the phase-control voltage V_(PC-2) ofFIG. 10B, but may occur before the turn-off time t_(off2) of thephase-control voltage V_(PC-2) of FIG. 10B. Therefore, the phase-controlvoltage V_(PC-3) of FIG. 10C may be said to overlap the phase-controlvoltage V_(PC-2) of FIG. 10B even though they may occur at differenttimes and in different line-cycles of the AC-line voltage. However, thephase-control voltage V_(PC-3) of FIG. 10C may not overlap thephase-control voltage V_(PC-2) of FIG. 10B.

The phase-control voltage V_(PC-3) may be characterized by a conductiontime T_(CON3). The conduction time T_(CON3) may correspond to the timewithin the positive half-cycle of the AC-line voltage when the loadcontrol device is providing voltage to the load. For example, theconduction time T_(CON3) may correspond to the time when a bidirectionalsemiconductor switch of the load control device is rendered conductive.The phase-control voltage V_(PC-3) may be characterized by a RMS voltagevalue V_(RMS-3). The RMS voltage value V_(RMS-3) may correspond to themagnitude of RMS voltage provided by the positive half-cycle of theAC-line voltage during the conduction time T_(CON3).

The phase-control signal of FIG. 10C may be characterized as operatingin an on-center phase control mode because the phase-control voltageV_(PC-3) comprises (e.g., overlaps) the midpoint 1002 of the positivehalf-line cycle of AC-line voltage and the midpoint 1002 of the positivehalf-line cycle AC-line voltage substantially corresponds to the halfwaypoint of the conduction time T_(CON3). Therefore, approximately an equalportion of the conduction time T_(CON3) occurs before the midpoint 1002as occurs after the midpoint 1002. A phase control signal operating inan on-center phase control mode may be characterized by a turn-on timethat is after the zero-crossing of the AC line voltage as the AC linevoltage goes from positive to negative and before the midpoint of ahalf-line cycle of the AC line voltage, and a turn-off time that isafter the midpoint of the half-line cycle of the AC line voltage andbefore next zero-crossing of the AC line voltage as the AC line voltagegoes from negative to positive, and where the midpoint substantiallycorresponds to the halfway point of the conduction time of the phasecontrol signal. A phase control signal operating in an on-center phasecontrol mode is not a forward phase control signal, a reverse phasecontrol signal, or a full conduction control signal.

Referring to FIG. 10D, a phase control signal operating in off-centerphase control mode is illustrated. The phase control signal of FIG. 10Dmay be characterized by the phase-control voltage V_(PC-4). The phasecontrol signal of FIG. 10D may be referred herein as the phase-controlvoltage V_(PC-4). The phase-control voltage V_(PC-4) may becharacterized by a turn-on time t_(on4) and a turn-off time t_(off4).The turn-on time t_(on4) may occur after a zero crossing of a positivehalf-line cycle of the AC line voltage, and before (e.g., as shown inFIG. 10D) or after a midpoint 1003 of the positive half-line cycle ofthe AC line voltage. The turn-off time t_(off4) may occur during thepositive half-line cycle of the AC-line voltage and before the negativezero-crossing of the AC-line voltage. For example, the turn-off timet_(off4) of the phase-control voltage V_(PC-4) may occur before, at, orafter (e.g., as shown in FIG. 10D) the midpoint 1003 of the positivehalf-cycle of AC-line voltage.

With respect to a positive half-line cycle of AC-line voltage, theturn-on time t_(on4) of the phase-control voltage V_(PC-4) may occurafter the turn-on time t_(on3) of the phase-control voltage V_(PC-3) ofFIG. 10C, but may occur before the turn-off time torn of thephase-control voltage V_(PC-3) of FIG. 10C. Therefore, the phase-controlvoltage V_(PC-4) of FIG. 10D may be said to overlap the phase-controlvoltage V_(PC-3) of FIG. 10C even though they may occur at differenttimes and in different line-cycles of the AC-line voltage. However, thephase-control voltage V_(PC-4) of FIG. 10D may not overlap thephase-control voltage V_(PC-3) of FIG. 10C.

The phase-control voltage V_(PC-4) may be characterized by a conductiontime T_(CON4). The conduction time T_(CON4) may correspond to the timewithin the positive half-cycle of the AC-line voltage when the loadcontrol device is providing voltage to the load. For example, theconduction time T_(CON4) may correspond to the time when a bidirectionalsemiconductor switch of the load control device is rendered conductive.The phase-control voltage V_(PC-4) may be characterized by a RMS voltagevalue V_(RMS-4). The RMS voltage value V_(RMS-4) may correspond to themagnitude of RMS voltage provided by the positive half-cycle of theAC-line voltage.

The phase-control signal of FIG. 10D may be characterized as operatingin an off-center phase control mode because the phase-control voltageV_(PC-4) comprises (e.g., overlaps) the midpoint 1003 of the positivehalf-line cycle of AC-line voltage, but the midpoint 1003 of thepositive half-line cycle AC-line voltage does not substantiallycorrespond to the halfway point of the conduction time T_(CON4). Thephase-control signal of FIG. 10D may be similar to the phase-controlsignal of FIG. 10B with the exception that the majority of theconduction time T_(CON4) of the phase-control signal of FIG. 10D isafter the midpoint 1003 of the positive half-line cycle AC-line voltage,while the majority of the conduction time T_(CON2) of the phase-controlsignal of FIG. 10B is before the midpoint 1001 of the positive half-linecycle AC-line voltage.

Referring to FIG. 10E, a phase control signal operating in forward phasecontrol mode is illustrated. The phase control signal of FIG. 10E may becharacterized by the phase-control voltage V_(PC-5). The phase controlsignal of FIG. 10E may be referred herein as the phase-control voltageV_(PC-5). The phase-control voltage V_(PC-) may be characterized by aturn-on time t_(on5) and a turn-off time t_(off5). The turn-on timet_(on5) may occur after a zero crossing of a positive half-line cycle ofthe AC line voltage. The turn-on time t_(on5) may occur before, at, orafter (e.g., as shown in FIG. 10E) a midpoint 1004 of the positivehalf-line cycle of the AC line voltage. The turn-off time t_(off5) mayoccur at, or substantially at, a zero crossing of the AC line voltage asthe AC line voltage goes from the positive half-line cycle to thenegative half-line cycle. For example, the turn-off time tom may besubstantially at the zero crossing of the AC line voltage when low-end(e.g., lowest 1-5%) and/or high-end portions (e.g., highest 1-5%) of thehalf-cycle of the AC line voltage are not utilized, for example, in someload control devices utilized for dimming lighting loads.

With respect to a positive half-line cycle of AC-line voltage, theturn-on time t_(on5) of the phase-control voltage V_(PC-5) may occurafter the turn-on time t_(on4) of the phase-control voltage V_(PC-4) ofFIG. 10D, but may occur before the turn-off time t_(off4) of thephase-control voltage V_(PC-4) of FIG. 10D. Therefore, the phase-controlvoltage V_(PC-5) of FIG. 10E may be said to overlap the phase-controlvoltage V_(PC-4) of FIG. 10D even though they may occur at differenttimes and in different line-cycles of the AC-line voltage. However, thephase-control voltage V_(PC-5) of FIG. 10E may not overlap thephase-control voltage V_(PC-4) of FIG. 10D.

The phase-control voltage V_(PC-5) may be characterized by a conductiontime T_(CON5). The conduction time T_(CON5) may correspond to the timewithin the positive half-cycle of the AC-line voltage when the loadcontrol device is providing voltage to the load. For example, theconduction time T_(CON5) may correspond to the time when a bidirectionalsemiconductor switch of the load control device is rendered conductive.The phase-control voltage V_(PC-5) may be characterized by a RMS voltagevalue V_(RMS-5). The RMS voltage value V_(RMS-5) may correspond to themagnitude of RMS voltage provided by the positive half-cycle of theAC-line voltage.

Two or more of the conduction times T_(CON1), T_(CON2), T_(CON3),T_(CON4), and T_(CON5) may be the same. For example, all of theconduction times T_(CON1), T_(CON2), T_(CON3), T_(CON4), and T_(CON5)may be the same. Two or more of the conduction times T_(CON1), T_(CON2),T_(CON3), T_(CON4), and T_(CON5) may be different. For example, all ofthe conduction times T_(CON1), T_(CON2), T_(CON3), T_(CON4), andT_(CON5) may be different.

Two or more of the RMS voltage values V_(RMS-1), V_(RMS-2), V_(RMS-3),V_(RMS-4), and V_(RMS-5) may be the same. For example, all of the RMSvoltage values V_(RMS-1), V_(RMS-2), V_(RMS-3), V_(RMS-4), and V_(RMS-5)may be the same. Two or more of the RMS voltage values V_(RMS-1),V_(RMS-2), V_(RMS-3), V_(RMS-4), and V_(RMS-5) may be different. Forexample, all of the RMS voltage values V_(RMS-1), V_(RMS-2), V_(RMS-3),V_(RMS-4), and V_(RMS-5) may be different.

The turn-on event (turn-on time) of a line-cycle of each successivephase-control signal may be equal to or greater than (e.g., later intime relative to the most recent zero-crossing) the turn-on event of theprevious line-cycle of the phase-control signal. For example, asdescribed with reference to FIGS. 10A-10E, the turn-on event t_(on2) ofthe phase-control signal of FIG. 10B may be greater than the turn-onevent t_(on1) of the phase-control signal of FIG. 10A. The turn-on eventt_(on3) of the phase-control signal of FIG. 10C may be greater than theturn-on event t_(on2) of the phase-control signal of FIG. 10B. Theturn-on event t_(on4) of the phase-control signal of FIG. 10D may begreater than the turn-on event t_(on3) of the phase-control signal ofFIG. 10C. The turn-on event t_(on5) of the phase-control signal of FIG.10E may be greater than the turn-on event t_(on4) of the phase-controlsignal of FIG. 10D.

The firing angle of a phase-control signal operating in a forwardphase-control mode may refer to a turn-on event, for example. The firingangle of a phase-control signal operating in a reverse phase-controlmode may refer to a turn-off event, for example. The firing angle of aphase-control signal operating in a center phase-control mode may referto a turn-on event and/or a turn-off event, for example.

The phase control signal of FIGS. 10A-10E may comprise correspondingnegative half-line cycles of the AC-line voltage that is symmetrical tothe positive half-line cycles of the AC-line voltage. Therefore,although described with reference to the positive half-line cycles ofthe AC-line voltage, the description of the negative half-line cycles ofthe phase control signals of FIGS. 10A-10E may be similarlycharacterized as the positive half-line cycles. However, the phasecontrol signals of FIGS. 10A-10B may comprise corresponding negativehalf-line cycles of the AC-line voltage that is not symmetrical to thepositive half-line cycles of the AC-line voltage.

Using a phase-control signal operating in a center phase control mode,such as an off-center phase control mode or an on-center phase controlmode, for example, may allow a load control device to operate (e.g.,dim) a variety of different loads (e.g., incandescent load, LED lamp,CFL load, etc.) under stable operating conditions. Some load types, suchas LED and CFL loads, for example, may require a bus capacitor to becharged to operate properly. If the bus capacitor is not charged, then acurrent spike may result, which may, for example, damage a component ofthe load. The bus capacitor may be charged using the peak voltages ofthe phase control signal. However, some load types, such as incandescentlamps, for example, may exhibit instability during dimming if an RMSvoltage value exceeding a threshold is provided to the load. Forexample, an incandescent lamp may flash on if an RMS voltage valueexceeding the threshold is provide to the load. Therefore, using aphase-control signal operating in a center phase control mode may allowthe load control device to provide peak voltages to the load to charge abus capacitor of the load (e.g., as quickly as possible), while stilllimiting the total RMS voltage provided to the load. Thus, using aphase-control signal operating in a center phase control mode may allowthe load control device operate (e.g., dim) a variety of different loadsunder more stable operating conditions, for example, by charging a buscapacitor of the load and limiting the total RMS voltage provided to theload.

An electrical load, such as an LED lamp, for example, may require acertain minimum voltage across it in order to turn on (e.g., emitlight). Once turned on, the load may be able to operate with an evenlower voltage (e.g., and thus provide a lower light intensity) than isrequired to turn the lamp on. A load control device may utilize aphase-control signal operating in a reverse phase-control mode, a centerphase-control mode, and a forward phase-control mode in a singleoperation, for example, to charge a bus capacitor of the load, providesufficient voltage across the load to turn it on, and operate the loadwhile allowing it to reach its lowest possible light output (low-end).

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

The invention claimed is:
 1. A load control device comprising: acontroller configured to control an amount of power delivered from analternating current (AC) power source to an electrical load, wherein ata transition of the electrical load from an off state to an on state,the controller is further configured to: provide a first line cycle of aphase control signal, the first line cycle characterized by a firstturn-on event and a first conduction time; subsequent to providing thefirst line cycle, provide a second line cycle of the phase controlsignal, the second line cycle characterized by a second turn-on eventand a second conduction time, the second turn-on event greater than thefirst turn-on event; subsequent to providing the second line cycle,provide a third line cycle of the phase control signal, the third linecycle characterized by a third turn-on event and a third conductiontime, the third turn-on event greater than the second turn-on event; andmaintain subsequent line cycles of the phase control signal inaccordance with the third turn-on event; wherein the first conductiontime, the second conduction time, and the third conduction time aresubstantially the same.
 2. The load control device of claim 1, whereinthe controller is further configured to: determine to operate theelectrical load using a forward phase control signal prior to providingthe first line cycle of the phase control signal.
 3. The load controldevice of claim 1, wherein the first phase control signal charges a buscapacitor of the electrical load.
 4. The load control device of claim 1,wherein the first phase control signal is a reverse phase controlsignal.
 5. The load control device of claim 1, wherein the second phasecontrol signal is a center phase control signal.
 6. The load controldevice of claim 1, wherein the third phase control signal is a forwardphase control signal.
 7. A load control device comprising: a controllerconfigured to control an amount of power delivered from an alternatingcurrent (AC) power source to an electrical load, wherein at a transitionof the electrical load from an off state to an on state, the controlleris further configured to: provide a first line cycle of a phase controlsignal, the first line cycle characterized by a first turn-on event anda first RMS voltage value; subsequent to providing the first line cycle,provide a second line cycle of the phase control signal, the second linecycle characterized by a second turn-on event and a second RMS voltagevalue, the second turn-on event greater than the first turn-on event;subsequent to providing the second line cycle, provide a third linecycle of the phase control signal, the third line cycle characterized bya third turn-on event and a third RMS voltage value, the third turn-onevent greater than the second turn-on event; and maintain subsequentline cycles of the phase control signal in accordance with the thirdturn-on event; wherein the first RMS voltage value, the second RMSvoltage value, and the third RMS voltage value are substantially thesame.
 8. The load control device of claim 7, wherein the controller isfurther configured to: determine to operate the electrical load using aforward phase control signal prior to providing the first line cycle ofthe phase control signal.
 9. The load control device of claim 7, whereinthe first phase control signal charges a bus capacitor of the electricalload.
 10. The load control device of claim 7, wherein the first phasecontrol signal is a reverse phase control signal.
 11. The load controldevice of claim 7, wherein the second phase control signal is a centerphase control signal.
 12. The load control device of claim 7, whereinthe third phase control signal is a forward phase control signal.
 13. Amethod for controlling an amount of power delivered from an alternatingcurrent (AC) power source to an electrical load by a load control deviceat a transition of the electrical load from an off state to an on state,the method comprising: providing a first line cycle of a phase controlsignal, the first line cycle characterized by a first turn-on event anda first conduction time; subsequent to providing the first line cycle,providing a second line cycle of the phase control signal, the secondline cycle characterized by a second turn-on event and a secondconduction time, the second turn-on event greater than the first turn-onevent; subsequent to providing the second line cycle, providing a thirdline cycle of the phase control signal, the third line cyclecharacterized by a third turn-on event and a third conduction time, thethird turn-on event greater than the second turn-on event; andmaintaining subsequent line cycles of the phase control signal inaccordance with the third turn-on event; wherein the first conductiontime, the second conduction time, and the third conduction time aresubstantially the same.
 14. The method of claim 13, further comprising:determining to operate the electrical load using a forward phase controlsignal prior to providing the first line cycle of the phase controlsignal.
 15. The method of claim 13, wherein the first phase controlsignal charges a bus capacitor of the electrical load.
 16. The method ofclaim 13, wherein the first phase control signal is a reverse phasecontrol signal.
 17. The method of claim 13, wherein the second phasecontrol signal is a center phase control signal.
 18. The method of claim13, wherein the third phase control signal is a forward phase controlsignal.
 19. A method for controlling an amount of power delivered froman alternating current (AC) power source to an electrical load by a loadcontrol device at a transition of the electrical load from an off stateto an on state, the method comprising: providing a first line cycle of aphase control signal, the first line cycle characterized by a firstturn-on event and a first RMS voltage value; subsequent to providing thefirst line cycle, providing a second line cycle of the phase controlsignal, the second line cycle characterized by a second turn-on eventand a second RMS voltage value, the second turn-on event greater thanthe first turn-on event; subsequent to providing the second line cycle,providing a third line cycle of the phase control signal, the third linecycle characterized by a third turn-on event and a third RMS voltagevalue, the third turn-on event greater than the second turn-on event;and maintain subsequent line cycles of the phase control signal inaccordance with the third turn-on event; wherein the first RMS voltagevalue, the second RMS voltage value, and the third RMS voltage value aresubstantially the same.
 20. The method of claim 19, further comprising:determining to operate the electrical load using a forward phase controlsignal prior to providing the first line cycle of the phase controlsignal.
 21. The method of claim 19, wherein the first phase controlsignal charges a bus capacitor of the electrical load.
 22. The method ofclaim 19, wherein the first phase control signal is a reverse phasecontrol signal.
 23. The method of claim 19, wherein the second phasecontrol signal is a center phase control signal.
 24. The method of claim19, wherein the third phase control signal is a forward phase controlsignal.